Rotatable mirrors are commonly used in laser printers. The mirrors are a series of flat mirror surfaces combined to form a regular polygon. Each of the flat surfaces, as it rotates, directs a laser beam reflected from the surface to sweep across a photoconductor. This is widely practiced.
This invention is directed to reducing the noise caused by the air driven by the corners of the polygon. Such noise has a strong pure tone (single frequency) component. This invention employs a partially surrounding wall and a second wall, the second wall being located to cancel noise. Such cancellation of noise is believed to occur by the second wall causing air currents which are out of phase with those caused by the mirror.
It is known to add walls which result in noise reduction. In a commercial printer which is prior art to this invention, a rib was added near to and over a cooling fan for a power supply. The rib extended over generally one-half of the fan and was positioned on the side opposite power supply toward which the fan drove air. Other judicious positioning of sound reducing walls are known to be widely used, although the exact form and location may in each case be the result of trial and error.